The internal environment of the body remains constant, within certain limits, despite changes that occur in the external environment. The control process that maintains conditions within these limits is known as hoemostasis.
The factors that are controlled include water balance, blood glucose concentration, blood pH, carbon dioxide concentration and body temperature.
Each of these has a ‘normal’ or set point although they may vary slightly above or below it. For example the normal body temperature for humans is about 37∘C.
In living cells, all the chemical reactions are controlled by enzymes. The enzymes are very sensitive to the conditions in which they work. A slight fall in temperature or a rise in acidity may slow down or stop an enzyme from working and thus prevent an important reaction from taking place in the cell.
The cell membrane controls the substance that enter and leave the cell, but it is the tissue fluid that supplies or removes the substances and it is therefore important to keep the composition of the tissue fluid as steady as possible. If the tissue fluid were to become too concentrated, it would withdraw water from the cells by osmosis and body would be dehydrated. If the tissue fluid were to become too dilute, the cells would take up tooo much water from it by osmosis and the tissues would become waterlogged and swollen.
Many systems in the body contribute to homeostasis. The obvious example is the kidneys, which remove substances that might poison the enzymes. The kidneys also control the level of salts, water and acids in the blood. The composition of the blood affects the tissue fluid which in turn affects the cells.
Another example of a homeostatic organ is the liver, which regulates the level of glucose in the blood. The liver stores any excess glucose as glycogen, or turns glycogen back into glucose if the concentration in the blood gets too low. The brain cells are very sensitive to the glucose concentration in the blood and if the level drops too far, they stop working properly, and the person becomes unconscious and will die unless glucose is injected in the blood system. This shows how important homeostasis is to the body.
The lungs play a part in homeostasis by keeping the concentration of oxygen and carbon dioxide in the blood at the best level for the cells chemical reactions, especially respiration.
The brain has overall control of the homeostatic processes in the body. It checks the composition of the blood flowing through it and if it is too warm too cold, too concentrated or has too little glucose, nerve impulses or hormones are sent to the organs concerned, causing them to make the necessary adjustments.
The skin and temperature control
Figure below shows a section through skin. In the basal layer some of the cells are continually dividing and pushing the older cells nearer the surface. Here they die and are shed at the same rate as they are replaced. The basal layer also contributes to the hair follicles. The dividing cells give rise to the hair.
There are specialised pigment cells in the basal layer and epidermis. These produce a black pigment, melanin which gives the skin its colour. The more melanin, the darker is the skin.
The thickness of the epidermis and the abundance of hairs vary in different parts of the body. See Figure across.
The dermis contains connective tissue with hair follicles, sebaceous glands, sweat glands, blood vessels and nerve endings. There is usually a layer of adipose tissue (a fat deposit) beneath the dermis.
The outermost layer of dead cells of the epidermis helps to reduce water loss and provides a barrier against bacteria. The pigment cells protect the skin from damage by the ultraviolet rays in sunlight. In white-skinned people, more melanin is produced in response to exposure to sunlight, giving rise to a tan.
Scattered throughout the skin are large numbers of tiny sense receptors, which give rise to sensations of touch, pressure, heat, cold and pain. These make us aware of changes in our surroundings and enable us to take action to avoid damage to recognise objects by touch and to manipulate objects with our hands.
The skin helps to keep the body temperature more or less constant. This is done by adjusting the flow of blood near the skin surface and by sweating. These process are described more fully below.
The two processes of heat gain and heat loss are normally in balance but any imbalance is corrected by a number of methods, including those described below.
- More blood flows near the surface of the skin, allowing more heat to be exchanged with the surroundings.
- Sweating – the sweat glands secrete sweat on to the skin surface. When this layer of liquid evaporates, it takes heat (latent heat) from the body and cools it down.
- Less blood flows near the surface of the skin, reducing the amount of heat loss to the surroundings.
- Sweat production stops – thus the heat lost by evaporation is reduced.
- Shivering – uncontrollable bursts of rapid muscular contraction in the limbs release heat as result of respiration in the muscles.
In these ways, the body temperature remains at about 37∘C. We also control our temperature by adding or removing clothing or deliberately taking exercise.
Whether we feel hot or cold depends on the sensory nerve endings in the skin, which respond to heat loss or gain. You cannot consciously detect changes in your core temperature. The brain plays a direct role in detecting any changes from normal by monitoring the temperature of the blood. A region called the hypothalamus contains a thermoregulatory centre in which temperature receptors detect temperature changes in the blood and co-ordinate a response to them. Temperature receptors are also present in the skin. They send information to the brain about temperature changes.
Normal human body temperature varies between 35.8∘C and 37.7∘C. Temperatures below 34∘C or above 40∘C, if maintained for long, ae considered dangerous. Different body regions, e.g. the hands, feet, head or internal organs, will be at different temperatures, but the core temperature, as measured with a thermometer under the tongue, will vary by only 1 or 2 degrees.
Heat is lost from the body surface by conduction, convection, radiation and evaporation. The amount of heat lost is reduced to an extent due to the insulating properties of adipose (fatty) tissue in the dermis. Some mammals living in extreme conditions, such as whales and seals, make much greater use of this: they have thick layers of blubber to reduce heat loss more effectively. Just how much insulation the blubber gives depends on the amount of water in the tissue: a smaller proportion of water and more fat provide better insulating properties.
Heat is gained, internally, from the process of respiration in the tissues and externally, from the surroundings or from the Sun.
The skin has another very important mechanism for maintaining a constant body temperature. This involves arterioles in the dermis of the skin, which can widen or narrow to allow more or less blood to flow near the skin surface through the blood capillaries.
Vasodilation – the widening of the arterioles in the dermis allows more warm blood to flow through blood capillaries near the skin surface and so lose more heat.
Vasoconstriction – narrowing (constriction) of the arterioles in the skin reduces the amount of warm blood flowing through blood capillaries near the surface.
The body’s responses to changes in core temperature
|Responses to a rise in body temperature||Responses to a fall in body temperature|
|Arterioles in the skin||dilate (widen) so that more blood flows to skin capillaries – excess heat from the core of the body is lost from the skin||narrow to restrict flow of warm blood to the skin capillaries – heat is retained in the body|
|Sweat glands||produce more sweat, which evaporates from the skin surface to cool it||cease production of sweat|
|Muscles||remain relaxed||muscular activity such as shivering generates heat|
|Metabolic rate||may decrease to minimise heat production||thyroxine increases metabolic rate|
Control of blood glucose levels
If the level of sugar in the blood falls, the islets release a hormone called glucagon in the bloodstream. Glucagon acts on the cells in the liver and causes them to convert some of their stored glycogen into glucose and so restore the blood sugar level.
Insulin has the opposite effect to glucagon. If the concentration of blood sugar increases (e.g. after a meal rich in carbohydrate), insulin is released from the islet cells. When the insulin reaches the liver it stimulates the liver cells to take up glucose from the blood and store it as glycogen.
Insulin has many other effects; it increases the uptake of glucose in all cells for use in respiration; it promotes the conversion of carbohydrates to fats and slows down the conversion of protein to carbohydrate.
All these changes have the effect of regulating the level of glucose in the blood to within narrow limits – a very important example of homeostasis.
The concentration of glucose in the blood of a person who has not eaten for 8 hours is usually between 90 and 100 mg 100 cm-3 but 2 hours later, the level returns to about 95 mg as the liver has converted the excess glucose to glycogen.
About 100 g glycogen is stored in the liver of a healthy man. If the concentration of glucose in the blood falls below about 80 mg 100 cm-3 blood, some of the glycogen stored in the liver is converted by enzyme action into glucose, which enters the circulation. If the blood sugar level rises above 160mg 100 cm-3, glucose is excreted by the kidneys.
A blood glucose level below 40 mg 100 cm-3 affects the brain cells adversely, leading to convulsions and coma. By helping to keep the glucose concentration between 80 and 150mg, the liver prevents these undesirable effects and so contributes to the homeostasis of the body.
If anything goes wrong with the production or function of insulin, the person will show the symptoms of diabetes.
The body’s responses to changes in blood glucose
|Responses to a rise in blood glucose above normal||Responses to a fall in blood glucose below normal|
|Pancreas||β cells in the pancreas produce the hormone insulin||α cells in the pancreas produce the hormone glucagon|
|Glucose uptake or release||insulin stimulates cells in the liver and muscles to take in glucose and convert it to glycogen and fat, which can be stored; inside the cells – blood glucose levels fall||glucagon stimulates the hydrolysis of glycogen to glucose in liver cells – glucose is released into the blood|